Heat transfer systems are used to transfer heat from one location (the heat source) to another location (the heat sink). Heat transfer systems can be used in terrestrial or extraterrestrial applications. For example, heat transfer systems may be integrated into satellite equipment that operates within zero- or low-gravity environments. As another example, heat transfer systems can be used in electronic equipment, which often requires cooling during operation.
Loop Heat Pipes (LHPs) and Capillary Pumped Loops (CPLs) are passive two-phase heat transfer systems. Each include an evaporator thermally coupled to the heat source, a condenser thermally coupled to the heat sink, fluid that flows between the evaporator and the condenser, and a fluid reservoir for expansion of the fluid. The fluid within the heat transfer system may be referred to as the working fluid. The evaporator includes a primary wick and a core that includes a fluid flow passage. Heat acquired by the evaporator is transported to and discharged by the condenser. These systems utilize capillary pressure developed in a fine-pored wick within the evaporator to promote circulation of working fluid from the evaporator to the condenser and back to the evaporator. The primary distinguishing characteristic between an LHP and a CPL is the location of the loop's reservoir, which is used to store excess fluid displaced from the loop during operation. In general, the reservoir of the CPL is located remotely from the evaporator, while the reservoir of an LHP is co-located with the evaporator.
Hybrid loops are two-phase heat transfer systems in which sweepage can be accomplished with a mechanical pump or passively through the use of a secondary (or auxiliary) evaporator.